The transmission of data, between a transmitter and a receiver which are separated by a transmission medium which disturbs their transmission, must be protected against these disturbances which give rise to errors at the receiving end.
The data can for example be supplemented with a simple parity bit or a relatively short codeword for detecting one or more errors in a block of data received and the receiver rerequests, if it is equipped for transmission, retransmission of the block in the event of an error.
In the case of the problem from which the present invention stems, radio transmission is involved, as in a GSM telephone network.
For radio transmissions, atmospheric glitches and those due to the relief are liable to disturb, sometimes very greatly, the transmissions. Therefore, it is necessary to be able to tolerate errors, without having to request retransmission, which would run the risk of being disturbed all the same. The useful data are then supplemented with redundancy data which make it possible not only to detect, up to a determined number, the errors in a data block, but also to locate and hence to correct, by inversion, the erroneous bits of the block.
Stated otherwise, taking the example of blocks of useful data which, for some of these blocks, differ from one another by just a single bit and may therefore be mistaken for one another in the event of an error in this bit at the receiving end, the adding of the redundancy bits, which form a particular signature for each block, allows these protected blocks to then differ by N bits.
If therefore one of the blocks thus protected is marred by from 1 to Nxe2x88x921 errors in determined bits which would tend to transform it into another block, it will be detected since the erroneous block will not exhibit the intended correlation between all its bits, which correlation is generated by the redundancy. If the number of erroneous bits does not exceed half the xe2x80x9cdistancexe2x80x9d of N bits separating, or differentiating, the block considered from the other blocks, a correction of these bits by virtue of the redundancy reestablishes the integrity of the block.
Thus, at the receiving end, and for each possible error-free block, the search for its representation is extended to a constellation of representations which differ therefrom only by from 1 to N/2 bits (erroneous), which erroneous representations it will be decided to recognize finally as being in reality the error-free block considered.
Intense radio disturbances of relatively long duration may however nearly interrupt any reception of a data block and therefore cause an error in a number of bits which is greater than that which the redundancy makes it possible to tolerate. In order to guard against the effects of these disturbances, the transmission of the block is spread over time by splitting it up into packets transmitted in successive bursts, each burst being able to include several packets, but belonging to different blocks.
Furthermore, the redundancy does not take the form of specific bits all of which would run the risk of being lost with a burst, that is to say not involving a juxtaposing of redundancy bits with the useful data bits but rather a global transformation, by an algorithm, of the representation of the block of useful data into another redundant representation, in one piece. The useful information and the redundancy are thus integrated, or indivisible in transmission, into the entirety of the new block and the redundancy therefore exists at every point of this new block of increased size.
The risk linked with the loss of a burst thus affects several different blocks rather than just a single block but, on the other hand, the transmission of each block is affected in only a much more limited manner by the loss of a burst and the redundancy is sufficient at the receiving end to reconstruct the data of the blocks transmitted, by an algorithm for extracting the useful data, which is the inverse of the transmission algorithm.
Such a mode of secure transmission is used for example in a GSM radiotelephony terminal. At the receiving end, the terminal receives a determined number of frames of bursts forming a block of data with redundancy. This redundancy nevertheless has the drawback of requiring logic processing of the bits according to the extraction algorithm, and hence of operating, and powering, a processing circuit so increasing the power consumption of the radio circuits. This is also true in respect of transmission. The same holds for the standby state in which the terminal receives a frame cyclically.
Now, the endurance of the supply to portable radio terminals is relatively limited.
In a general manner, the present invention aims to limit, at least at the receiving end, the excess consumption, electrical or in terms of machine time, linked with redundancy, without however losing the protection offered by the latter.
For this purpose, the invention relates to a process for transmitting data by radio pathway between a transmitter rig and a receiver rig, through a disturbing transmission medium, in which:
the data to be transmitted are grouped into blocks,
redundancy information is added to each block to be transmitted, and
the block is split up into a determined number of bit packets and,
at the receiving end,
an attempt is made to reconstruct the data block from packets received, by utilizing the redundancy information, a process in which:
a quality is estimated in respect of the transmission of data by the radio pathway and
a minimum number of packets whereby it is possible to reconstruct each block with the aid of the redundancy information is determined therefrom,
a number of packets of the block which is at least equal to the minimum number is transmitted and,
at the receiving end, the attempted reconstruction of the block is performed by investigating a number of packets corresponding to the minimum number.
Thus, having estimated the quality of the transmission pathway, it is possible to choose to transmit a number of packets which is less than the determined number and greater than or equal to the minimum number estimated to be necessary.
In a first case, of transmission of the minimum number of packets, the receiver rig will, a priori, investigate all the packets received, although it is not however excluded from attempting the reconstruction on the basis of a still lesser number if the actual quality is better than that scheduled. Stated otherwise, the correspondence alluded to above can in general be an equality between the minimum number and the actual number of packets investigated, but a variation, in one direction or in the other, in the actual quality of transmission can however give rise to a modulation of the number of packets investigated with respect to the number considered, a priori, as the minimum sufficient.
The concept of the invention, which consists ultimately in discarding the packets estimated to be superfluous, can be applied exclusively at the transmission end, but its application simultaneously benefits the transmitter rig, whose radio power circuits in particular will operate less often, and the receiver rig, at reduced traffic. In fact, the volume of data processed in the receiver is limited to a strict minimum and circuits performing the analysis can thus be kept quiescent for longer or else they can be assigned to other tasks. Additionally, even if the reconstruction of the block, on the basis of a reduced number of packets, is more complex, it is possible to save energy on account of the non-investigation, during radio reception or processing, of all the packets of the block.
In a second case, of transmission of the determined number of packets, the concept of the invention is then applied at the receiving end only, where the estimation of quality is utilized, with the advantages mentioned above.
Finally, the mixed case or cases of transmission of a number of packets which is greater than the minimum number and less than the determined number has, to a lesser degree, the advantages of the first case, but on the other hand makes it possible to tolerate an error in the estimation of the quality of the link since the receiver rig has available at least one additional packet as compared with the estimated minimum, as has been alluded to above.
It will be noted that the invention applies advantageously to radio transmissions, but not exclusively. It will be understood that the data transmission considered designates a digital transmission, without concerning the significance of the data themselves, that is to say it may in particular involve digitized voice signals.